1. Field of the Invention
This invention relates to the field of decontaminating soils, groundwater and other compositions or substances that contain unwanted charged species. Specifically, the invention relates to options for remediation of soils contaminated with heavy metal ions, including soils wherein the contamination is located in the vadose (unsaturated) zone overlaying groundwater.
2. Description of the Related Art
According to the method of this invention, in situ electrokinesis is employed to drive contaminants toward an electrode positioned in the soil. This electrode (typically an anode, in the case in which contaminant plumes to be remediated include negatively charged contaminant ions and particles) is positioned in a hole in the soil with, in the preferred embodiment, activated carbon partially or completely filling the hole around the electrode.
Various methods of electrokinetic remediation of soil containing charged contaminant species have been described in patents and elsewhere in the scientific literature. Fundamentally, the concept relies on the principle that charged species exhibit mobility in soil, both as a result of diffusion and in response to electrical phenomena. This mobility is mediated, in part, by the presence of moisture in the soil. Charged contaminant particles may be either dissolved or partially soluble in soil moisture. For the most part, the contaminants of concern bear overall negative charges. This is because negatively charged species (for example, chromate ions and other anionic species) often are harmful to the physiology of humans and animals. In principle, though, electrokinetic methodologies would be effective in concentrating and removing positively charged species, as well. In at least one embodiment of the present invention, positively charge species may be removed from soil. Typically, however, negatively charged species are the target for removal. Several examples of existing methods, illustrating aspects of the current state of the art of electrokinetic remediation of soils, are summarized below.
In U.S. Pat. No. 5,405,509, "Remediation of a Bulk Source by Electropotential Ion Transport Using a Host Receptor Matrix", remediation of soil, concrete and groundwater is described according to a process using ion exchange. In that method, an electrode is placed in the ground surface generally near or at the center of contamination, and electrical currents are used to drive contaminants to it. A "host receptor matrix" or HRM comprising polymeric ion exchange resins is placed around an electrode and both captures the contaminant ions as they migrate toward the electrode and buffers hydrogen or hydroxyl ions generated at the electrode. The adsorption capacities of such resins, however, can be limited and further adversely affected by heating of soils occurring as a result of current flowing through the soils. Additionally, the presence of significant amounts of water enhances performance of the method in the '509 patent, and in fact, performance is optimized when this method is used in soils that are nearly saturated with water.
As will be described in detail below, in contrast to the method of the '509 patent, the method of the present invention takes advantage of a low pH environment in proximity to the electrode for instances wherein anionic species are to be collected and removed. Moreover, in the method of the present invention, there are no resins, such as are used in the '509 patent, that react adversely to heat (if heat is generated) and the method operates satisfactorily in soil well below the point of moisture saturation.
Another technique is described in Schrieber, C. F., "ELGARD LIDA Deep Anode Groundbed Design and Installation Guidelines", Specification number ELG-DG, rev. A, January 1994, ELGARD Corp., Sugar Land, Tex. This technique involves using petroleum coke as conductive fill in cathodic protection anode installations. Large-scale cathodic protection anodes may be constructed by hanging a string of iridium coated titanium anodes in an open borehole and filling the hole with petroleum coke. The purpose of the petroleum coke is fundamentally to provide even current distribution to the surrounding soil. It is not used in the Schrieber reference to distribute electrical potential within the bed, itself, and thereby enhance contaminant collection. While the resistivity of the petroleum coke when used in a particle bed is similar to that of a bed of moist activated carbon (in the range of approximately 2 to 50 ohm-cm), the Schrieber reference does not capitalize on this feature in the way it is exploited in the present invention, as will be described below. Moreover, petroleum coke, used by Schrieber, does not exhibit adsorbent characteristics comparable to activated carbon. The bed material in the present invention offers distinct advantages both in terms of its adsorptive and electrical characteristics.
In another patented methodology, described in U.S. Pat. No. 5,398,756, "In-Situ Remediation of Contaminated Soils", a treatment zone is defined within a region of contaminated soil. Electrodes are placed outside the region, and electrical currents are used to drive contaminants through the treatment zone where they are either destroyed or captured using, for example, biological or physicochemical contaminant treatment modalities. In the '756 patent, electroosmosis and/or electromigration cause the flow of contaminants into and through a treatment zone into which material for treating contaminants is introduced.
In contrast to the method of the '756 patent, the present invention does not use electrical potentials to drive contaminants through an in situ treatment zone, per se. Rather, electrokinetics is employed to move contaminant species toward and into a bed containing activated carbon or similar adsorptive material that exhibits favorable electrical characteristics that optimize movement of charged particles though the bed. The contaminants are captured in the bed where the resulting mixture of adsorptive material and contaminants is easily removed. The '756 patent does not concern concentrating contaminants in a bed containing material easily removed and disposed of, nor does it teach the advantageous electrical features associated with the materials used in the method of the present invention.
A previous patent by the inventors of the present invention described and claimed an electrokinetic electrode assembly for use in extraction of soil contaminants. U.S. Pat. No. 5,435,895 "Electrokinetic Electrode System for Extraction of Soil Contaminants from Unsaturated Soils" described an assembly wherein a liquid comprising electrolyte solution, pure water, and soil water is retained within a housing to which an electrode is mounted. Engineered fluid flow takes place, subject to vacuum control, whereby contaminants are extracted by means of an electrode system while electrolyte and pure water are recycled.
The present invention offers a simplified method that does not require either engineered fluid flow or electrolytes. Instead, as will be described in detail below, the method of the present invention is largely passive, relying on the physics of electrokinesis and the favorable electrical and physical characteristics of the materials used.
Finally for purposes of this summary, there is mention in the literature of using activated carbon as an adsorbent for chromium (IV). An example is Perez-Candela, M., J. M. Martin-Martinez and R. Torregrosa-Macia, "Chromium (VI) Removal with Activated Carbons", Wat. Res., 29, 9, 2174-2180, 1995. Activated carbon has long been recognized as an adsorbent capable of removing organic compounds from water. During the past decade, studies have demonstrated that activated carbon is also capable of removing heavy metals from solution. The Perez-Candela, et al. reference mentioned here reports that chromate adsorption capacity of some activated carbons is as high as 3.5 g Cr/g carbon at a pH of 1. The present invention does not, however, rely exclusively on the adsorbent capacity described in the Perez-Candela, et al. report and others. Rather, as will be explained in detail below, the moderately conductive nature of carbon acts to enhance the efficacy of activated carbon for contaminant removal in the context of the method of the present invention. This is over and above the effects that may be attributed to the adsorbent capabilities of the activated carbon.
Each of the treatment approaches described exhibit certain benefits in treatment of contaminated soils and water. There remains, however, a need for an improved electrokinetic soil remediation method that takes advantage of naturally occurring low pH conditions near the anode functioning in an environment where water is present, and that exploits the electrical characteristics of moderately conductive materials to improve capture of charged contaminant species. Additionally, there remains a need for such a remediation method that yields a stable and easily disposable final waste form.